You asked a very good question.

Long ago when the idea of fast food was first being expanded thru franchising, most places that served French fries in the United States used a russet potato, which they cut by hand (later by wall mounted or table mounted fry cutters such as these.)  By the way, the very best potato cutter in my opinion is also the one that costs the most, about $300 called the Keen Kutter.  Many independent and emerging chains use the Russet Burbank variety from Idaho as it has consistently high solids or starch content and fries up crisp and golden.

To make a perfect fry takes some special efforts, here are some tips: All Things French Fried Potatoes